Add builder-method for snippet choice

[okmttdhr]

This PR fixes #43643

[msftclas]

All CLA requirements met.

[jrieken]

lgtm. thanks so far. one last thing that's left is to add the signature to the type in vscode.d.ts

[jrieken]

Looking good - one last thing: To expose this to extension authors, copy the new signature to the type declaration in vscode.d.ts, e.g here

vscode/src/vs/vscode.d.ts

Line 2979 in 58479e8

appendVariable(name: string, defaultValue: string | ((snippet: SnippetString) => any)): SnippetString;